This application claims priority form German Application Nos. DE 101 28 079.3 filed on Jun. 9, 2001 and DE 101 33 800.7 filed on Jul. 11, 2001.
1. Field of the Invention
The present invention relates to a device for detecting breakage of leno threads in leno selvedge devices on looms, the leno selvedge forming device being provided with at least two arms for guiding at least two leno threads, the threads being twistable together by virtue of the rotation of the arms.
The invention is also directed to a loom with heald frames, with at least one leno device for forming a leno selvedge which is reversible in its direction of rotation and with a facility for detecting breakage of the leno threads.
2. Description of the Prior Art
On its two lateral edges, a woven fabric is provided with what is termed a xe2x80x9cleno selvedgexe2x80x9d. Such a leno selvedge prevents the fabric from fraying. Various apparatuses for forming such a leno selvedge have become known; EP 0 674 031 for example discloses a rotational leno device for looms which is provided with a so-called doup disc having opposite eyelets for guiding the leno threads. A so-called xe2x80x9cfull leno selvedgexe2x80x9d can be formed with this prior art device in that the two leno threads are twisted together about the weft. In order to undo the twisting on the feeding side, it is necessary to reverse the direction of rotation after a certain number of revolutions.
Furthermore, DE 196 47 495 C1 describes a device for forming a full leno selvedge which is characterized by a driven rotating body with diagonally opposite arms, said arms being provided at their ends with eyelets for passing the leno threads therethrough. This device also needs to have the direction of rotation reversed after a certain number of revolutions in order for the threads to be untwisted on the feeding side.
It is known to use so-called xe2x80x9cwarp thread monitorsxe2x80x9d for detecting the breakage of the leno threads. Such warp thread monitors are arranged in the back shed between the leno selvedge apparatus and the bearing of the bobbins. Said warp thread monitors are configured as metallic, electrically conductive lamellae that hang from the leno threads.
If a thread breaks, the lamella located on the thread slumps by virtue of its own weight, thus closing an electrical contact, which stops the drive of the loom.
With various threads, monitoring the leno threads by means of lamellae proved impossible. With thin or delicate threads such as fiber glass threads for example, the strain exerted by the lamellae may be so high that they break under the very weight of the lamellae. Therefore, it has become known to monitor the leno threads by means of light barriers, scanners and the like. However, these systems are characterized by poor reliability because of their susceptibility to dust; also, the reaction times are too long.
More specifically when using the leno devices mentioned herein above provided with a facility to reverse the direction of rotation, i.e. with leno devices by means of which the leno threads are twisted on the feeding side, i.e., in the back shed, there is the risk, in utilizing warp thread monitors in the form of lamellae, that the lamellae will not fall down, even upon breakage of the thread, more specifically because the cotton threads are strongly sticking together, thus being capable of still carrying the lamella even in the broken condition of the thread. There is the risk that the broken thread is pulled, together with the other, still intact thread, through the eyelet thereof. Even when the direction of rotation is reversed, i.e., when the leno threads are untwisted on the feeding side, the heald will not fall off.
It is therefore the object of the invention to provide a device of the type mentioned herein above by means of which it is possible to reliably detect thread breakage even on leno devices that are provided with arms and which is reversible in its direction of rotation, i.e., on leno devices that cause the leno threads to be twisted on the feeding side, said threads having to be untwisted again by reversing the direction of rotation.
With a known leno device for forming a leno selvedge in a fabric provided with at least two arms for guiding at least two leno threads, the leno threads being twistable together by virtue of the rotation of the arms, the solution to this object is to suggest that the device for detecting thread breakage is provided with a facility for determining the natural oscillations of the arms. This solution is based on the following consideration:
The arms of the leno device are tensioned by the leno threads that are guided in the eyelets of the arms at the end thereof. While rotating, the arms execute a substantially uniform motion of rotation. The oscillating behavior of the arms is attenuated when the motion of rotation is stopped, this attenuation being effected by the tense guiding of the leno threads in the eyelets of the arms. If the leno thread breaks, the arm, through the eyelet of which no leno thread is guided any longer, is not subjected to any load, thus being without attenuation. As a result thereof, when the corresponding motion of rotation comes to an end, this arm is set oscillating with an oscillation that considerably differs from the oscillation or the pattern of oscillations the arm is subjected to under the load of the leno thread on stopping. The natural oscillating behavior of the arms or the pattern of oscillations resulting therefrom permits to directly assert the condition of the leno threads.
The essential point is that the facility for detecting the oscillating behavior of the arms is not contacting the arms, thus operating in a contactless manner so that the mass of the arms remains unchanged. The mass of the arms is to be kept low if the required high acceleration values are to be realized by the leno device.
More specifically, there is provided that the facility for detecting the oscillating behavior of the arms is provided, in the region thereof, with means for producing a magnetic field, a sensor unit for detecting changes in the magnetic field occasioned by the motion of the magnetically conductive arms of the leno device being provided for and an evaluation unit being provided that compares the oscillation spectra of the natural oscillations produced by the arms when the thread is broken and when it is intact, a signal delivery unit being arranged downstream of said evaluation unit. The rotation of the magnetically conductive arms allows the magnetic field to vary, the extent of variation of the magnetic field reflecting the oscillating behavior of the arms. That is to say that the pattern of oscillations corresponding to the motion of the arms accordingly produces a corresponding signal in the coil, or in the Hall sensor. These signals, which correspond to the patterns of oscillations of the arms under the condition of a broken or an intact thread, are next compared in the evaluation unit, an assertion being made, upon comparison, about whether the leno thread is broken or intact. In function of the result of said comparison, a signal delivery unit sends a signal to the drive of the loom for the purpose of stopping the loom if necessary. A variant is conceivable in which the arms are magnetized as well or in which permanent magnets for producing a magnetic field are arranged on said arms.
The very sensor unit can be provided with means allowing for inductive or capacitive operation and adapted to detect the oscillation of the arms of the leno device. Means allowing for capacitive operation and adapted to detect oscillations or the pattern of oscillations are a capacitor, a voltage change occurring in the capacitor at a certain frequency in function of the motion of the arms. It is also an object of the invention to provide a loom characterized by a device having one or several features of the device for detecting thread breakage described herein above, said device for detecting thread breakage being advantageously arranged in the front shed between the leno device and the heald frames.
Another subject matter of the invention is a loom, more specifically provided with heald frames, with at least one leno device which is reversible in its direction of rotation and with a facility for detecting breakage of the leno threads. As already explained herein above, it has become known to monitor breakage of the leno threads in the region of the back shed. This involves different difficulties and it has particularly to be stressed that in case of threads that are strongly sticking together, like e.g. cotton threads, it is not made certain that, even when the thread has broken in the region of the back shed, lamellae configured as warp thread monitors will indeed fall down, thus causing the drive of the loom to stop as desired.
It is therefore the object of the invention to arrange, on a loom of the type mentioned herein above and provided with a leno device which is reversible in its direction of rotation, a device for detecting breakage of leno threads in such a manner that the operation of the facility for detecting breakage of leno threads is not impaired by the twisting of the thread in the back shed.
The solution to this object in accordance with the invention is to arrange the device for detecting thread breakage in the front shed, more specifically between the heald frames and the leno device. In the front shed, undesirable twisting of the threads without weft does not take place in front of the heald frames. Accordingly, the device for detecting thread breakage can be arranged there without its function being affected in any way by the twisting of the leno threads, as it is the case when it is located in the back shed.
More specifically, there is provided that the device for detecting thread breakage is not connected to the leno threads, i.e., it is configured as a device operated in a contactless manner. Conceivable are sensors operated in a contactless manner in particular like for example light barriers, scanners, and the like. Such a sensor unit for detecting thread breakage is advantageously connected to an evaluation unit and/or directly with a signal delivery unit, the drive of the loom being controllable, i.e., in case of doubt, stoppable by said signal delivery unit.
The invention is explained herein after in more detail with reference to the drawing.